Amphipathic macromolecules have been synthesized as models for membrane proteins. Fatty acids were covalently linked to dextran derivatives of various molecular weights, and various amounts of hapten (TNP) and fluorophore (rhodamine or fluorescein) were also linked to the molecule. These molecules adsorb to all artificial and cell membranes tested. On planar lipid bilayers their lateral diffusion has been measured and patching induced upon addition of anti-TNP. On lymphocytes, these molecules patch and cap rapidly if exposed to anti-TNP; such capping is blocked by metabolic inhibitors. When adsorbed to the surface of lymphoid cells, these TNP molecules do not elicit the production of killer T lymphocytes nor are TNP-dextran target cells recognized or destroyed by killer T cells which do kill "TNP-self" target cells, i.e., cells whose surface antigens are modified by the reagent TNBS.